1-28 ResistanceRESISTANCEis the opposition to current flow. It is measured in ohms and is represented by the letter R. Formulas for resistance: Voltage VOLTAGEexists when a charge exists between two bodies. When a one coulomb charge exists, one unit of electrical potential energy is created. This is called a difference of potential, an electromotive force, or a voltage. It is measured in volts and represented by the letter E. Formulas for voltage: